Apparatus for producing foamed, molded thermoplastic articles and articles produced thereby

ABSTRACT

Method and apparatus for producing a molded article of a foamed thermoplastic resin and articles produced thereby. A molten mass of an expandable thermoplastic resin is accumulated in an accumulator while it is prevented from foaming. A quantity of the accumulated formable melt is extruded through a die orifice having a shape adapted to reflect the shape of the desire finished product. The extruded foamable melt commences foaming as it comes in contact with the atmosphere. As the extruded thermoplastic resin foams, it is pulled vertically downward from the die orifice by gravity. The downward-hanging, foamed, thermoplastic material is captured between the halves of a vertically-oriented mold before the foaming expansion of the foamable melt has been completed. The foamed thermoplastic material is compressed by the vertically-oriented mold into the desired shape. The foamed thermoplastic material may be formed into intricate articulations within the female mold portion by gases emitted from jets in the male mold portion or by venting or drawing off gases disposed in the female mold portion. The mold halves may also be configured to confine any foamed resin flash to the bottom portion of the molded article.

This is a divisional of application Ser. No. 08/019,825 filed Feb. 19,1993, now U.S. Pat. No. 5,246,976 which is a divisional application ofSer. No. 07/689,533, filed Apr. 23, 1991, now U.S. Pat. No. 5,202,069.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, in general, to foamed, molded,uncrosslinked thermoplastic articles. More specifically, method andapparatus for producing foamed, molded, thermoplastic shoe midsoles aredisclosed.

2. Description of the Prior Art

In the production of foamed articles of thermoplastic resin, threemethods are generally known (1) the bead molding method; (2) theinjection molding method; and (3) the extrusion method. Under the beadmolding method, pre-foamed particles are placed in a mold cavity andheated to encourage further expansion and fuse-bonding of the particles.The bead molding method is disadvantageous because it requires two ormore processing steps and traces or marks of the beads are left in theresulting molded article.

Under the injection molding method, a molten mass of a foamable resin isinjected into a mold cavity and the injected resin is allowed to expandin the mold. This method requires a high injection pressure, however,and hence, a large and strong molding apparatus capable of withstandingthe pressure. Moreover, the injection molding can only provide anexpansion ratio (i.e., density reduction from resin to foam) of at mostabout 2.

In the extrusion method, a molten, expandable resin is extruded througha die. The method may only be used to produce foamed articles of asimple shape such as sheet- or rod-like products, however.

One application where the above-noted problems of the foaming/moldingprior art are especially visible is in the production of athletic shoemidsoles (i.e., the material between the shoe upper and theground-contacting outer sole). Shoe midsoles have been characterized asthe most important portion of athletic footwear. The rather significantforces generated by runners as they run, especially in the ball,forefoot and heel regions of the foot, must be largely absorbed by theshoe midsole. Furthermore, the midsole preferentially is also capable ofreturning a significant portion of the runner's energy through his/herbody as the shoe contacts the ground, creating a beneficial sensation ofspringiness. Athletic shoe midsoles must also be able to withstand thelarge number of compression and return cycles generated by, for example,long-distance runners, without jeopardizing the weight bearing andcushioning capacity of the shoe midsole.

Specifically, material utilized for an athletic shoe midsole mustexhibit the requisite levels of hardness, resiliency and compressivestrength. Hardness is commonly measured by, for example, an ASKER Chardness tester (or dorometer). The hardness tester calculates thehardness of a test specimen from the measured depth of penetration of anindentor of predetermined geometry into the specimen (once a state ofbalance is reached between the resistance force of the specimen and theforce applied to the indentor). To be suitable for use as an athleticshoe midsole, thermoplastic foamed material must exhibit a hardness of30 to 70 ASKER C, and preferably exhibits a hardness of about 40 to 55ASKER C.

The resiliency of a material may be quantified by measuring thematerial's energy return ratio. In general, the energy return ratio isobtained by dropping an object onto the material and measuring how highthe object bounces back (e.g., a perfect spring would have an energyreturn ratio of 1.00). The methodology of measuring a material's energyreturn ratio is discussed in detail in U.S. Pat. No. 4,984,376, which ishereby incorporated by reference, at column 10, lines 37 to 64. To besuitable for use as an athletic shoe midsole, a material should exhibitan energy return ratio of at least 0.20 (using the method of measuringenergy return ratio disclosed in the ASTM bulletin number D-2632-79). Byway of comparison, under this testing procedure, foamed thermosettingpolyurethane exhibits a energy return ratio of about 0.25 to 0.30 andfoamed HYTREL® (a polyester elastomer manufactured by E.I. du Pont deNemours and Co. of Wilmington, Del.) exhibits an energy return ratio ofabout 0.50 or more.

Compressive strength is measured by gradually compressing a flat sampleof material (e.g., a cube with a 10 cm×10 cm (or 1 inch by 1 inch) topsurface) and measuring the pressure needed to compress the sample agiven proportion of its original height (e.g., 10%, 25% and 50%).Compressive strength is measured in kilo Pascals (kPa), or pounds persquare inch (psi). Preferred materials for athletic shoe midsoles shouldexhibit a compressive strength of about 48 to 138 kPa (7 to 20 psi) at10% compression, 117 to 207 kPa (17 to 30 psi) at 25% compression and248 to 379 kPa (36 to 55 psi) at 50% compression.

Another important criteria which any proposed athletic shoe midsolematerial must meet is the material's specific gravity. Specific gravityrelates to, and in some senses, grows out of the previously discussedproperties. To be suitable for use as an athletic shoe midsole, amaterial must have a specific gravity of about 0.5 gm/cm³ or less.Preferred midsole materials have a specific gravity of about 0.3 gm/cm³or less. This restriction, in turn, limits the methods which may be usedto form the midsole. For example, injection molding may typically onlybe used with materials having higher specific gravities than thosesuitable for use as athletic shoe midsoles (e.g., about 0.8 gm/c³). Iflower density materials are injection molded, the material will oftennot foam uniformly, thereby causing broken cells within the foamedproduct.

In addressing these concerns, the athletic footwear industry hasdeveloped a variety of different solutions. For example, many shoemidsoles are currently made of crosslinked EVA (ethylene vinyl acetate).Crosslinked EVA exhibits good durability, but since it is a crosslinkedmaterial, EVA generates a large amount of non-recyclable waste materialduring processing. Furthermore, production of midsoles from crosslinkedEVA normally requires several processing steps (see, e.g., U.S. Pat. No.4,900,490, which is hereby incorporated by reference). For example,after a plank of EVA is produced, the plank must be skived (i.e., cutalong its height to form two or more separate, thinner planks).Thereafter, the plank is cut into plugs having the approximateconfiguration of the desired midsole. The plugs of EVA are then insertedinto molds and compression molded. The plugs are purposely cut slightlyoversize relative to the molds to encourage the material to adapt to anyconfigurations present within the mold.

The compression molding step also re-forms a skin over the open cells ofmaterial which were exposed when the plank of EVA was skived. Thisprocessing methodology is both multi-step and time-consuming (e.g., 5 to10 minutes per compression cycle-i.e., heating to seal the skived EVAand allowing the re-compressed EVA to cool).

Other currently-available processes also exhibit several problems. Forexample, in producing shoe midsoles from thermoplastic material (e.g.,polyester elastomer), multi-step processing is still the norm. Forexample, a large piece of thermoplastic material is extruded.Thereafter, the material is skived, die cut into plugs of approximatelythe desired size and the plugs of material are subjected to secondarycompression molding to form designs in the material and to create acell-enclosing skin over the cut areas of foam.

When uncrosslinked thermoplastic materials are utilized, the largeamounts of waste material generated by this type of process may at leastbe recycled (with crosslinked material, the excess material cannot bereprocessed and must be discarded), but even with uncrosslinkedmaterials, these multiple processing steps still mandate that largeamounts of labor be expended in producing each foamed article.Furthermore, since the thermoplastic material is normally fully foamedwhen it is subjected to the secondary, skin-forming compression molding,it is difficult to produce articles having intricate areas and/or shapesof raised material (e.g., company logos on the sides of athletic shoemidsoles). Conventional foaming methods also have difficulty producingfoamed material having substantially uniform density and cell structurethroughout the article (e.g., in the expansion process of U.S. Pat. No.4,806,293, which is hereby incorporated by reference, the expandingmaterial may fold over on itself, thereby forming (YMP) seams in thefinished article).

The present method and apparatus, on the other hand, favorably resolvethese problems and suboptimizations inherent in the prior art byproviding a one-step process for producing low-density foamed articlesfrom uncrosslinked (and hence recyclable) thermoplastic material wherebyuniform density is maintained, foam cell integrity is maintained andeven intricate designs in the female mold section may be reproduced inthe shoe innersole.

SUMMARY OF THE INVENTION

In general, any known thermoplastic resin which is capable of beingfoamed to low density (e.g., 0.5 gm/cm³ or less) may be used as a rawmaterial in the process of the present invention. The foamability ofsuitable resins may alternatively be quantified by noting that the resinshould be capable of producing a density reduction of at least 0.5(i.e., comparing the unfoamed and foamed forms of the resin).Illustrative of suitable thermoplastic resins are olefins such aspolyethylenes, polypropylenes and copolymers thereof; styrene resinssuch as polystyrenes; polycarbonate resins; and thermoplasticpolyurethanes and copolyetherester elastomers. The method of the presentinvention may be used with a wide variety of materials, the choice of aparticular thermoplastic resin for a particular application will dependupon the particular article or properties desired to be produced.

The thermoplastic resin exemplified above is homogenously commingledwith a blowing agent at a temperature higher than the melting point ofthe resin under a pressurized condition to obtain a molten, expandablethermoplastic resin composition. Both decomposition-type blowing agentsand solvent-type blowing agents may be used for the purpose of thepresent invention. Examples of the solvent-type blowing agents includecycloparaffins such as cyclobutane and cyclopentane; aliphatichydrocarbons such as propane, butanes, pentanes, hexanes and heptanes;and halogenated hydrocarbons such a trichlorofluoromethane,dichlorodifluoromethane, methyl chloride, methylene chloride,dichlorotetrafluoroethane, tetrafluoroethane, tetrafluorochloroethane,trifluorochloroethane, pentafluoroethane, trifluoroethane,difluorochloroethane, ethyl chloride, trifluoropropane, difluoropropaneand octafluoropropane. Examples of decomposition-type blowing agentsinclude gypsum, hydrated aluminas, azodicarbonamide, mixtures of sodiumbicarbonate and citric acid, and sodium borohydrate. In general, a widevariety of blowing agents may be used in the present invention, theoptimal blowing agent for a particular application of the invention willdepend upon the type of resin being utilized and the type of articlebeing formed (and the desired article's performance parameters, e.g.,hardness, resiliency, compressiveness and specific gravity).

The molten, expandable resin composition may further contain nucleatingagents, e.g., to regulate cell size within the foam. Suitable nucleatingagents are known from the prior art. For example, talc, calciumcarbonate, calcium sulfate, diatomaceous earth, magnesium carbonate,magnesium hydroxide, magnesium sulfate, clay and barium sulfate may beuseful in particular applications. The nucleating agents are generallyused in amounts of from 0.5 to 5 percent by weight, preferably from 0.5to 2 percent by weight, based on the weight of the resin.

The molten, expandable resin composition may further containconventional additives in the usual amounts, e.g., pigments, dyes,fillers, flameproofing agents, antistatic agents, stabilizers,lubricants, plasticizers and nucleating agents.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail withreference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of an extruder/accumulatorapparatus suitable for use with the method of the present invention.;

FIG. 2 is a side plan view of an alternate extruder/accumulatorarrangement having a vertical gate;

FIG. 3 is an elevated perspective view of a mold apparatus of thepresent invention showing the male and female mold sections closed;

FIG. 4 is an elevated perspective view of a mold apparatus of thepresent invention showing the male and female mold sections open;

FIG. 5 is an elevated perspective view, partially in section to show thevents disposed in the female mold section, of a mold apparatus of thepresent invention showing a quantity of molten, expandable thermoplasticmaterial being ejected (e.g., from an extruder and/or accumulator) andbeginning to hang down between the male and female mold sections.

FIG. 6 is an elevated perspective view of a mold apparatus of thepresent invention showing the mold section open and showing a femalemold section having vents disposed therein;

FIG. 7 is a schematic representation of an alternate configuration ofthe present invention wherein multiple molds are arranged on a circularcarousel;

FIG. 8 is an elevated perspective view of a molded foam midsole producedby an apparatus of the invention;

FIG. 9 is a side view, in cross-section, of a molded foam midsoleproduced by an apparatus of the invention wherein the flash may beremoved by hand; and

FIG. 10 is a side view, in cross-section, of a molded foam midsoleproduced by an apparatus of the invention wherein the flash is limitedto the bottom of the midsole.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The general process for preparing suitable foamable thermoplastic resinis outlined schematically in FIG. 1. The raw materials, such asthermoplastic resin pellets and, optionally, an expansion aid introducedfrom a hopper 2 are commingled with a blowing agent supplied through aline 2a in a mixing zone, designated generally as 1. The mixing isconducted at a temperature higher than the melting point of thethermoplastic resin and under a pressure to obtain a foamable melt.Customarily employed temperature and pressure conditions for mixing andmelting thermoplastic resins (which are dependent upon the resin,blowing agent, additives, etc., used), are utilized in the presentinvention. The mixing is generally effected by means of an extruder 3provided with a screw 4. Any extrusion system capable of producing foammay suitably be used.

The foamable melt of the expandable resin composition thus prepared inthe mixing zone 1 is fed to an accumulator 5 through a passage 6 andaccumulated therein. With some foam compositions, it may be advantageousto feed the foamable melt into a cooling zone before feeding thematerial into the accumulator 5. For some compositions, pre-accumulatorcooling is helpful in achieving the optimum foaming temperature. A setof tandem extruders (one for mixing and a second to allow forpre-accumulator cooling) may also be used. Alternatively, in addition tomixing and melting zones, a portion of the extruder 3 may be configuredto cool the mixture.

During the accumulation of the molten resin, the inside of theaccumulator is maintained at a temperature and a pressure under whichsubstantially no expansion of the molten resin is caused. Generally, thefoamable melt is maintained at a temperature approximately the same asthe melt exiting the extruder and lower than that in the mixing zone 1.For example, when a low density polyethylene is used, the accumulator ispreferably maintained at a temperature of 88° to 116° C. (190° to 240°F.) and a pressure of 1034 to 6895 kPa (150 to 1000 psi). Particulartemperature and pressure conditions will vary widely, however, dependingupon the type of material being foamed and the type of article beingproduced. Heating and/or cooling units may be employed in the mixingand/or accumulator portions of the apparatus to assure that appropriatetemperature conditions are maintained prior to foaming.

When a predetermined amount of the foamable melt has accumulated in theaccumulator 5, it is rapidly discharged from the accumulator 5, througha die 7, into an atmosphere maintained at a pressure lower than that inthe accumulator 5 (generally ambient pressure) so that the extrudedfoamable melt 10 commences expanding. To expedite the discharge of thefoamable melt, it is preferable to use a means for forcibly dischargingthe molten resin. Thus, in the embodiment shown in FIG. 1, theaccumulator 5 has a cylindrical shape and is provided with a gate (notshown) for opening and closing the aperture of the die 7 and with a ram8 reciprocally slidably disposed therewithin. The ram 8 is secured to apiston rod 9 of a hydraulic means (not shown). Rapid discharge of theaccumulated foamable melt is preferred to assure that all portions ofthe extruded material are equally foamed and cooled (e.g., in anathletic shoe midsole, to assure that both the heel and the toe portionsof the discharged material are equally foamed and cooled) and topreclude premature formation of a skin on the discharged material.Preferably, the accumulator 5 is capable of intermittently dischargingmaterial at a rate of about 500 Kg/hr or more.

The ram 8 is preferably operated in the following manner. The moltenresin prepared in the mixing zone 1 is continuously introduced underpressure into the accumulator 5 through the passage 6 while the apertureof the die 7 is closed by the gate. The pressure is transmitted to theram 8 so that the ram 8 is gradually slid as the molten resin isaccumulated within the accumulator 5. When the foamable melt accumulatedin the accumulator 5 reaches a predetermined volume, the hydraulic meansis actuated and, simultaneously, the gate is opened. Thus, the pistonrod 9 is driven to advance the ram 8 toward the die 7, thereby ejectingthe accumulated foamable melt through the die 7 into the atmosphere.Thereafter, the gate is closed and the pressure of the hydraulic meansis released so as to decrease the pressing force of the ram 8 to apredetermined level.

The pressure inside the accumulator 5 is controlled so as to remainwithin a range such that the foaming of the molten resin is prevented.This can be performed by controlling the pressure exerted by thehydraulic means and applied to the ram 8.

Alternatively, the molten material may be ejected into the atmospheredirectly from the extruder 3 (if sufficient discharge rates are attainedwhereby foaming of the material within the die is prevented anddifferential cooling of the extruded material is prevented--e.g., toprevent differential cooling in the heel and toe regions of a shoemidsole). In a second alternate embodiment, the material (after leavingthe extruder 3) may be first passed through a second, cooling, extruderbefore being passed into the accumulator 5.

A preferred example of the accumulator as described above is disclosedin U.S. Pat. No. 4,323,528, the disclosure of which is herebyincorporated by reference. FIG. 2 shows an alternateextruder/accumulator arrangement having a horizontal gate 70, wherebythe material leaving the accumulator 5 moves vertically.

Upon being extruded from the accumulator 5, the thermoplastic material10 begins to foam. As shown in FIG. 5, the material is preferablyallowed to hang down vertically from the die 7 in an unsupportedfashion. In FIG. 5, the material 10 is shown being ejected from a pipe11 (e.g., connected to an accumulator 5), but in appropriate situations,the thermoplastic material 10 could be extruded directly from theextruder 3. Furthermore, the pipe 11 may be horizontally disposed (asshown in FIGS. 5 and 7) or vertically disposed (e.g., or the materialmay be extruded vertically-downwardly from the accumulator 5 directly,e.g., FIG. 2). Since the thermoplastic material 10 is preferably beingextruded between the mold halves of a vertically-oriented moldapparatus, however, the discharge pipe 11 is preferablyvertically-oriented to limit the tendency of the ejected thermoplasticmaterial 10 to curl (i.e., reflecting a flow direction memory of thenon-vertical components of the movement of the material 10 within thepipe 11).

Most thermoplastic materials 10 become almost fully foamed essentiallyimmediately after being extruded from the accumulator 5. Therefore, nopurposeful period of free-fall (i.e., between the pipe 11 and the mold20) is normally necessary. Rather, in order to limit the amount ofexcess material being extruded (i.e., in excess of the amount ofmaterial required to fill the mold) the ejected material 10 ispreferably captured by the male 21 and female 22 mold sections as soonas feasible after being extruded. Quickly capturing the ejected materialwithin the mold 20 helps to preclude the formation of a foamcell-covering "skin" on the extruded material (since the skin maypreclude the material from filling intricate shapes in the moldsections) and helps to maintain a homogenous level of expansion andsolidification within the extruded material 10 before molding (e.g.,from toe to heel of a shoe midsole). From a practical viewpoint,however, it may be best to allow some free-fall of the extruded material10 before molding (i.e., distance between the pipe 11 and mold 20) tohelp limit the fouling of the outside of the mold 20 by excessthermoplastic material (since uncrosslinked thermosetting material ispreferably used with the inventive process, however, any excessmaterial, e.g., on the outside of the mold, can at least be reprocessedand reused). In some situations, however, (e.g., with certain types ofresins, or when making some types of articles) it may be best to allowthe ejected material a longer period of free-fall before molding iscommenced.

The shape of the extruded material 10 may be largely controlled byutilization of an appropriately shaped die 7. For example, a rectangulardie may be used to produce extruded material having a generallyrectangular cross-section and a round die may be used to produce agenerally cylindrically-shaped mass of extruded material. Morepreferably, the die shape and extrusion rate may be coordinated toproduce an extruded mass of material 10 having the desiredconfiguration. For example, if a round die is employed and theaccumulator 5 is programmed to extrude the material 10 in a(comparatively-speaking) slow-fast-slow sequence, due to the fact thatmost thermoplastic materials foam more vigorously when extruded athigher rates, the material will form a narrow-wide-narrow cylindricalmass of material suitable for molding into a football. Different dieconfigurations and extrusion rates may be used to produce extrudedmasses of material of widely-varying shapes.

Once the requisite amount of foamable melt material has been ejectedfrom the pipe 11 (or alternatively, from the accumulator 5 or extruder 3directly), e.g., about 100-150 grams for an average size shoemidsole-which is preferably accomplished in about two seconds or less ofejection time (the weight of the ejected material and preferred ejectiontime depending upon the density of the foamable melt material beingused), the mold sections are closed around the ejected material 10(often called a "parison" in the industry). As the male 21 and female 22mold sections are moved together (e.g., hydraulically or by any othersuitable method) they enclose therebetween a mold cavity 23 (e.g.,having the form of the desired shoe midsole). As the mold sectionsclose, they capture a portion of the parison of thermoplastic material10 within the mold cavity 23. Alternatively, more than two mold sectionsmay be used to constitute the mold cavity 23.

As discussed above, with many thermoplastic materials, the material isnearly fully foamed upon ejection, but any minor amount of remainingfoaming/expansion when the material is captured within the mold cavity23 may actually be helpful in conforming the shape of the thermoplasticmaterial into the shape of the mold cavity 23, and especially, infilling in any depressions or void areas within the female mold section22, e.g., representing company logos on the sides of the shoe midsoles.In part because of the short amount of time between ejection and captureof the parison of thermoplastic material, the material within the moldcavity is still easily formable when it is captured and may be somewhatcompressed by the action of the mold sections to further facilitate theformation of intricately-shaped molded articles.

In an alternate embodiment (see FIG. 5), the male mold section 21 isprovided with a plurality of jets 24 for emitting gases (e.g., air)under pressure. The gas emitted from these jets 24 further helps toinsure that the ejected thermoplastic material 10 fully fills any voidsor depressed areas within the female mold section 22 by forcing thematerial into the female mold section 22. In order to allow the actionof the jets 24 to effectively act upon (i.e., move) the material 10within the mold cavity 23, the mold cavity should include an air-tightseal (e.g., provided by rubber gaskets disposed between the moldsections). The jets 24 may also be helpful in limiting the formation ofexcess material (or "flash") around the molded article (which must betrimmed from the molded article after the process is complete).

In a second alternate embodiment (see, FIG. 5, and especially, FIG. 6),one or both of the mold sections 21, 22 (preferably the female moldsection 22) is provided with a plurality of vents 25, whereby gases mayescape/be drawn out of the mold cavity 23 as the parison is compressedby the two mold sections. Hence, in this embodiment, vents 25 also helpto insure that the ejected thermoplastic material 10 fully fills anyvoids or depressed areas within the mold sections 21, 22. As notedabove, although the vents 25 are preferably disposed within the femalemold section 22, vents 25 may be disposed in either or both moldsections 21, 22. Vents 25 should not be disposed in a mold section whichalso has jets 24, however.

The foamed thermoplastic material 10 is preferably maintained within themold cavity 23 until the thermoplastic material has solidified into theform of the desired article. If the pressure on the thermoplasticmaterial is maintained for too little time, (i.e., the mold sections areseparated too soon) post-release expansion of the material may occur(e.g., causing cells within the foam to pop, and thereafter, causingvoids to form in the molded article). If pressure is maintained on thethermoplastic material 10 for too long, however, the foam within someareas of the molded article may collapse (causing sink marks). Thepreferred amount of molding time will vary according to the materialbeing utilized, the mass of material being molded and the type ofarticle being formed. For example, in forming an athletic shoe midsolefrom HYTREL®, the foamed material is preferably maintained within themold for about 2 to 3 minutes.

After the thermoplastic foam has been maintained within the mold cavity23 for an appropriate length of time, the two mold sections 21, 22 areseparated and the molded foam material (e.g., shoe midsole) is removed.Since the parisons of material ejected from the pipe 11 (or accumulator5, or extruder 3) will always be of slightly different sizes, there willalways be some excess material which must be trimmed from the finishedmolded article (e.g., with a shoe midsole of desired finished weight of105-110 grams, the molded material present when the mold is openedcommonly has a total weight of 135-139 grams). This excess material,e.g., around the edges of the molded article, is known in the industryas "flash" (e.g., in FIG. 8, flash 50). The flash may be easily trimmedfrom around the edges of the midsole without destroying the outersurface, or "skin" of the molded article. Preferably, the mold sections21, 22 are configured to minimize the amount of flash generated duringprocessing, however, to limit the amount of wasted material. The moldsections 21, 22 are preferably configured to meet at, for example, thebottom portion of the molded article (e.g., shoe midsole), whereby theflash may be trimmed from only one side (i.e., the bottom) of the moldedarticle (see, FIGS. 9 and 10). In this way, for example, if jets 24 areused, any imperfections created in the surface of the molded article bythe jets 24 may be eliminated when the flash is trimmed. Morepreferably, the mold sections 21, 22 are configured to produce a flatbottom surface on the molded article whereby the flash may be trimmedoff with a single horizontal cut (see, FIG. 10--both flash 50 and bottomimperfections, e.g., caused by jets 24, may be eliminated in one cut).Most preferably, the mold sections 21, 22 are configured to form a sharpline of intersection (or "part" line) therebetween (e.g., through use ofa male mold section having only a short ridge around the outline of themidsole--see, FIG. 5, thereby forming a wedge 51 between the midsole andthe flash 50, facilitating separation of the midsole and flash) whereby,after the molded article is released from the mold cavity 23, the flashwill be attached to the molded article by only a thin bridge of materialwhich may be easily severed by hand by ripping the flash from the moldedarticle (see, FIG. 9). Furthermore, since the process of the presentinvention preferably utilizes uncrosslinked thermoplastic material, anyflash produced may be reprocessed/remelted and incorporated into futuremolded products.

Given the length of time the mold 20 must remain closed as the parisonof ejected foamed thermoplastic material 10 solidifies therein (i.e., tothe point where, respectively, both post-release expansion and collapseof foam cells are avoided--e.g., for a shoe midsole formed from HYTREL®,about 2-3 minutes), the inventive apparatus is more preferablyconfigured to allow for more continuous utilization of theextruder/accumulator, i.e., a plurality of parisons of thermoplasticmaterial being ejected during any one molding cycle. A variety ofapparatus configurations may prove beneficial in this regard. Forexample, FIGS. 3-5 show the mold 20 mounted on rails 30, whereby afterone parison of thermoplastic material has been ejected into one mold 20,the mold may be slid out from under the extruder/accumulator pipe 11 andanother (perhaps differently-sized) mold 20 may be positioned beneaththe pipe 11 to receive a second parison 10 of thermoplastic material.The accumulator 5 may also be adjusted or preprogrammed to vary thequantity of material ejected per parison and/or the speed at which thematerial is ejected. In another preferred embodiment, a plurality ofmolds 20 may be mounted on a circular carousel (see, FIG. 7). Forexample, if a particular molded article has a mass such that theavailable accumulator can produce ten parisons of material of therequisite size to produce the article per minute and the articlesrequire one minute of cooling time within the mold cavity to solidify, acarousel having ten molds may profitably be employed. Also, to insureconsistent mold temperatures during the preferred continuous moldingprocess, the mold 20 also has associated therewith a temperature controlmechanism 40 for heating or cooling the mold 20 as required.

The inventive apparatus and process may be further understood throughreference to the following non-limiting examples.

EXAMPLES Foamed Athletic Shoe Midsoles Materials

A preferred thermoplastic multi-block copolymer elastomer used in someof the examples was a copolyetherester elastomer sold by DuPont underthe trademark HYTREL®, grade 4078W. Depending upon the desiredproperties, however, a variety of other elastomers may also be used withthe apparatus and process of the present invention. For example,ethylene vinylacetate (EVA), SANTOPRENE® (a thermoplastic elastomer madeby Monsanto, Co.), KRATON® (a styrene-butadiene elastomer made by ShellChemical Co.), PELLETHANE (a thermoplastic polyurethane made by the DowChemical Co.), and a variety of other materials such as copolyetheramideester may also be used in appropriate circumstances. As discussed above,regardless of the particular material being foamed, it is important thatthe requisite levels of hardness, resiliency, compressive strength andspecific gravity be attained. For example, in the production of athleticshoe midsoles, the foamed material should exhibit at hardness of 30 to70 ASKER C (preferably about 40 to 55 ASKER C), an energy return ratio(i.e., resiliency) of at least 0.20 (under ASTM method D-2632-79), acompressive strength of 48 to 138 kPa (7 to 20 psi) at 10% compression,117 to 207 kPa (17 to 30 psi) at 25% compression, and 248 to 379 kPa (36to 55 psi) at 50% compression. Finally, to be suitable as a midsole, aelastomer should be foamable to a specific gravity of about 0.5 gm/cm³or less.

Both decomposition-type blowing agents and solvent-type blowing agentsmay be used in the present examples. Traditional nucleating agents andother foaming materials are also employed as hereinafter described infurther detail.

Foam Preparation

The foams described in the following examples were prepared in a 7.62 cm(3 inch) diameter, 48:1 (length:diameter) extruder. The extruder wasequipped with an apparatus for injecting therein foaming agents and theforward portion of the extruder barrel was jacketed for cooling usingcirculating water. The extruder was attached to a foam accumulator,e.g., as described in U.S. Pat. No. 4,323,528. The accumulator wasequipped with a piston for ejecting (extruding) the foamable meltthrough a closable die. The speed of the piston was variable to providevarious extrusion rates. The use of an accumulator is not necessary toproduce foams of large cross-sections with a large extruder. However,its use was required with the relatively small foam extruder used in theexamples, which, by itself, would be incapable of producing foams oflarge cross-section. The use of a relatively small extruder alsoconserved raw materials as the foamable melt was intermittently extrudedat rates of about 454 to 2268 kg/hr (1000 to 5000 lbs/hr) (preferablyabout 1134 kg/hr (2500 lbs/hour)) while the actual output rate of theextruder was about 54.4 kg/hr (120 lbs/hr). Within the accumulator, thefoamable material is preferably maintained at about 3448 kPa (500 psi).

EXAMPLE 1

The foam accumulator was equipped with a radially-configured die havingan aperture with cord length of 3.42 cm (1.345 inches), an arc length of3.81 cm (1.50 inches) and a gap of 0.279 cm (0.110 inches) [thedimensions of the particular die orifice utilized will depend upon thearticle being manufactured, the type of material being foamed, theejection rate of the material, etc.]. Because the thermoplastic resinsutilized in the present invention are commonly hygroscopic, the resin(e.g., in the form of pellets) was desiccated before being fed into theextruder. This desiccation of the resin may be accomplished in thehopper or before the resin is introduced into the extruder. Normally,contacting the resin with air which has been exposed to a desiccant(e.g., silica gel) and heated for about 2 hours at 93° C. (200° F.)adequately desiccates the resin [with HYTREL®, however, it is preferableto desiccate the resin with air which has been heated to 107° C. (225°F.)]. The thermoplastic multiblock copolymer elastomer HYTREL® 4078 Wwas mixed into the hopper of a single-screw extruder. The elastomer wasmixed with about 0.33% (by weight of total mix) of HYDROCEROL™ CF-40(for cell size control-"Hydrocerol" is an encapsulated mixture of sodiumbicarbonate, citric acid and citric acid salts which liberates carbondioxide and water under elevated temperatures in the extruder) and about2 1/2% (by weight) white color masterbatch. The foaming agent,isobutane, was injected into the extruder at a rate of about 0.32 kg perhour (0.7 lbs/hr). The output of the extruder was about 54.4 kg per hour(120 lbs/hr). After the foaming agent was injected, it was mixed intothe polymer and then the mixture was cooled to the proper foamingtemperature, about 174° C. (about 345° F.). The foamable melt exitingthe extruder was transferred under pressure to the accumulator where itwas stored and released intermittently at a rate of 1134 kg per hour(2500 lbs/hour).

Upon opening of the accumulator gate, a parison of material wasextruded. The amount of material extruded will depend upon the type ofarticle being formed and how well tuned the apparatus is. In the case ofa shoe midsole, it is expected that approximately 135 to 139 grams ofmaterial will be trapped between the mold halves of the apparatus (i.e.,forming the molded article and the flash). Upon extrusion, the foamablemelt commenced foaming and hung downward from the die orifice. Theextruded material was captured between the male and female mold sectionsof a vertically-oriented mold. The male and female sections of the moldenclosed the extruded material within a mold cavity in the shape of ashoe midsole. The extruded material was maintained in the mold cavityfor 23/4 minutes (165 seconds) until the shoe midsole was formed. Afterthe flash was trimmed, the midsole weighed about 105 to 110 grams.

EXAMPLE 2

The same materials used in Example 1 were used in this example. Theapparatus used to mold the ejected, foamed thermoplastic resin included,in the male mold section, a plurality of jets. The jets were used toemit pressurized air (e.g., at about 138 kPa (20 psi)) against thematerial captured within the mold cavity, thereby urging the material tofill even intricately-shaped recesses (e.g., brand logos) within thefemale mold section. The mold sections were gasketed with rubber toprevent gas from escaping from between the mold sections. The moldsections were configured to join near the bottom of the midsole, wherebythe flash could be removed in a single, horizontal cut. Insofar as thishorizontal cut shears cells within the molded foam material, it may evenbe beneficial in the production of athletic shoe midsoles since thishorizontal surface is subsequently bonded to other materials (e.g., theshoe outsole) and the open cells may assist subsequent glue bonding.

EXAMPLE 3

The same materials used in Example 1 were used in this example. Theapparatus use to mold the ejected, foamed thermoplastic resin included,in the female mold section, a plurality of apertures for venting ordrawing-off gases from the material captured within the mold cavity,thereby urging the material to fill even intricately-shaped recesseswithin the female mold section.

The vents are preferably matched to a particular type of mold, wherebyall excess gasses may be removed from the mold cavity. For example, inthe case of forming an athletic shoe midsole, if the mold sections areconfigured to join together (or form a "part line") at the bottom of themidsole, vents may preferably be positioned on the top of the midsoleand in any fins disposed in the midsole (e.g., on the sides of themidsole). Venting is especially helpful in indented areas of the femalemold section, because air may otherwise easily become trapped in theseareas and prevent the foamed material from filling the recessed areas,thereby causing gaps in the fins of the finished articles. On the otherhand, for example, if the part line between the two mold sections isconfigured to be formed at the top of the midsole, it may be unnecessaryto provide vents in all of the fins (i.e., since some of the air whichwould otherwise be trapped in the vents may escape the mold cavitythrough the part line).

The positioning of the part line and the utilization of vents arepreferably balanced to achieve the overall goal of minimizing the amountof gas which is trapped inside the mold cavity when the thermoplasticmaterial is molded. In general, positioning the part line at the bottomof the midsole makes it easier to trim the flash from the midsole,whereas positioning the part line near the top of the midsole decreasesthe number of vents which must be used to evacuate the gas from the moldcavity (since the part line may vent much of the gas trapped in themidsoles indented fins).

EXAMPLE 4

The same materials used in Example 1 were used in this example. In orderto facilitate removal of the flash, the mold sections were configured toform only a very sharp part line just above the top of the shoe midsole,whereby the flash was only very tenuously attached to the moldedmidsole. The sharpness of the part line permitted the flash to be peeledaway from the molded article by hand.

EXAMPLE 5

The same equipment used in Example 1 was used in this example. Theelastomer utilized was PELLETHANE® Series 2102-90a, a polyesterpolycaprolactone manufactured by The Dow Chemical Company of Midland,Mich. After the resin was desiccated by being contacted for two hourswith air which had been heated to 93° C. (200° F.) and passed through adessicant, the PELLETHANE® Series 2102-90a was mixed with the blowingagent (isobutane--which constituted about 11 percent by weight of themix) and the nucleating agent (talc--which constituted about 0.5 percentby weight of the mixture or less). The output of the extruder was about50.8 kg per hour (112 lbs/hour) of material. After the blowing agent wasinjected, it was mixed into the polymer and then the mixture was cooledto the proper foaming temperature, about 204° C. to 207° C. (400° F. to405° F.). The foamable melt exiting the extruder was transferred underpressure to the accumulator where it was stored at a pressure of about1551 to 1724 kPa (225 to 250 psi) and released intermittently at a rateof about 19996 kPa (2900 lbs/hour).

A portion of the extruded thermoplastic material weighing approximately115 to 120 grams was captured between the male and female mold sectionsof a vertically-oriented mold. The mold sections enclosed the ejectedmaterial within a mold cavity in the shape of a shoe midsole. Theejected material was maintained in the mold cavity (the mold cavity ofthis example was the same size as that utilized in Example 1) for about23/4 minutes (165 seconds) until the midsole had solidified. After beingreleased from the mold and having the flash trimmed, the midsole weighedabout 85 to 90 grams.

Like the midsole produced in Example 1 (and like the midsoles producedby all of the other examples as well) the article of moldedthermoplastic material produced by the inventive process in this examplewas capable of performing as an athletic shoe midsole (i.e., hadsufficient hardness, resiliency, compressive strength and specificgravity). Relative to the midsole produced in Example 1, the midsoleproduced in this example had a lower density, a lower resiliency and ahigher rigidity per mass of material. Hence, depending upon the desiredproperties of the finished article sought in a particular application,the apparatus and process of the present invention may profitably beutilized with a wide variety of materials.

I claim:
 1. An apparatus for producing a foamed, molded articlecomprising:(a) means for preparing foamable melt comprising a moltenmass of a foamable thermoplastic resin at a temperature and a pressureat which said foamable melt is prevented from foaming, said preparingmeans including a die orifice whereby when said orifice is opened, aportion of said foamable melt is extruded and whereby said extrudedmaterial is suspended from said preparing means and said foamable meltbecomes almost fully foamed essentially immediately after saidextrusion; (b) means for capturing and compressing at least a portion ofthe extruded foamable melt, said means for capturing and compressingcomprising an openable and closeable mold, said mold including at leasttwo substantially vertically-oriented mold sections movable relativelyto one another, and when said mold is closed, said mold (c) a pluralityof jets disposed in at least one of said mold sections for emitting apressurized gas, whereby the tendency of said foam to fill cavitieswithin the other mold section or sections is augmented; and (d) saidmold cavity being positionable adjacent to at least a portion of saidsuspended foam and said mold cavity being in the shape of said foamed,molded article.
 2. The apparatus of claim 1, wherein said means forpreparing said foamable melt is an extruder.
 3. The apparatus of claim1, wherein said foamable melt is formed in an extruder, transferred toan accumulator and then extruded through said orifice.
 4. The apparatusof claim 1, further comprising a plurality of vents disposed in at leastone of said mold sections, whereby gases may be vented or drawn off fromsaid mold cavity, whereby the tendency of said thermoplastic resin tofill cavities within said at least one mold section is augmented.
 5. Theapparatus of claim 1, wherein said mold cavity is in a shape of a shoemidsole.
 6. The apparatus of claim 5, wherein said mold cavity comprisesmeans for restricting the formation of any flash attached to said shoemidsole to the bottom of said shoe midsole.
 7. The apparatus of claim 1,further comprising a plurality of said openable and closeable molds,whereby sequential masses of said foamable material may be extruded fromsaid preparing means, and the molding of said sequential masses ofmaterial may be commenced before the molding of a first mass of extrudedmaterial has been completed.
 8. The apparatus of claim 7, wherein saidplurality of molds are disposed on a carousel.